Abstract
We examine 15 orbits of Hubble Space Telescope observations of the nearby post-core-collapsed globular cluster NGC 6397 using narrowband photometry in Hα with broadband photometry in V, R, and I. We recover the candidate helium white dwarfs (He WDs) recently discovered by Cool et al., of which one was examined spectroscopically by Edmonds et al., along with a sequence of these objects extending down to the magnitude limit of the survey. We also find a sequence of stars with weak Hα emission and magnitudes characteristic of BY Draconis stars; their offset above the main sequence also suggests they are probable binaries. The radial distributions for both the He WD and BY Dra candidates are significantly more centrally concentrated than the main-sequence stars, suggesting that indeed both populations are in binary systems.
Highlights
Knowledge of stellar dynamics and stellar interactions in globular clusters has advanced significantly over the past decade with the discovery of multiple cataclysmic variables (CVs) in each of three clusters with the Hubble Space Telescope (HST; Cool et al 1995; Grindlay et al 1995; Bailyn et al 1996; Knigge et al 2001) and millisecond pulsars (MSPs) with increasingly sensitive radio surveys (Camilo et al 2000; D’Amico et al 2001)
As the stripped cores ofgiants, helium white dwarfs (He white dwarfs (WDs)) are a natural outcome of compact binary evolution
Three objects in NGC 6397 were first identified by Cool et al (1998, hereafter CGC) as stars significantly bluer than the main sequence (MS) but lacking the flicker variations generally seen in CVs
Summary
Knowledge of stellar dynamics and stellar interactions in globular clusters has advanced significantly over the past decade with the discovery of multiple cataclysmic variables (CVs) in each of three clusters with the Hubble Space Telescope (HST; Cool et al 1995; Grindlay et al 1995; Bailyn et al 1996; Knigge et al 2001) and millisecond pulsars (MSPs) with increasingly sensitive radio surveys (Camilo et al 2000; D’Amico et al 2001). Together with the increasingly rich populations of low-luminosity X-ray sources in globulars (e.g., Verbunt & Hasinger 1998), these systems all are manifestations of compact binaries containing white dwarfs (WDs) or neutron stars (NSs). They are markers of the formation and evolution of hard binaries that can, collectively, have profound effects on the dynamical evolution of the clusters themselves (Hut et al 1992). The new source populations, and the MSPs, have sparked new theoretical understanding of the formation and evolution of the compact binaries needed (e.g., Rasio, Pfahl, & Rappaport 2000) Another marker of compact binary populations in globulars are helium white dwarfs (He WDs). Follow-up HST/Faint Object Spectrograph observations by Edmonds et al (1999, hereafter EGC) of one of these “nonflickerers” (NF 2) yielded a spectrum consistent with an He WD and 247 ע50 km sϪ1 Doppler shift—suggesting it is in a binary with a dark companion, most probably a massive WD or an NS and by implication that the
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